Some vehicle powertrain systems incorporating automatic transmissions include the option to automatically shift the transmission to a neutral gear when the parking brake is applied. This is useful, for example, in a refuse packer vehicle that may make frequent stops where the driver exits the vehicle for a short time while leaving the engine running. Coming to a stop and engaging the parking brake will cause the transmission to be automatically shifted to a neutral gear, thus saving the driver the time that would be required to shift to a neutral gear using the transmission control selector. When the driver returns to the vehicle, releasing the parking brake will cause the transmission to be automatically shifted to the previous drive gear, thus saving the driver from using the transmission range selector to shift the transmission back into a drive gear. This system requires dual redundant wires indicating the status of the parking brake be applied to the transmission control unit. As long as the service brake is not indicated to be in error, if both of the redundant wires indicate that the parking brake has been released, the transmission will automatically be shifted to the previous drive gear.
Therefore, there remains room for improvements in the provision of the automatic functions in an automatic transmission.